Electrostatic printers of the type described in WO 93/11866 eject charged solid particles dispersed in a chemically inert, insulating carrier fluid by using an applied electric field to first concentrate and then eject the solid particles. A single printhead will typically comprise a number of ejectors, each of which can be made to eject a volume of ink depending on the voltage applied at the ejection locations.
Various printhead designs have been described in the prior art, such as those in WO 93/11866, WO 97/27058, WO 97/27056, WO 98/32609, WO 98/42515, WO 01/30576 and WO 03/101741.
In order to achieve consistent ejection of ink from the printhead, precise control of the static pressure of the ink is required at the ejection locations. The ink pressure may be controlled through a combination of air pressure and gravity by using a reservoir with a weir which feeds the printheads, the difference in height between the top of the weir and the ejection locations determining the total depth of ink and, thus, the pressure due to gravity. A printhead in which the ejectors are at differing heights will experience varying ink pressures across its length which will cause a corresponding variation in ejection performance.
To print on a cylindrical object, one or more printheads may be aligned such that their ejectors are arranged parallel to the longitudinal axis of the object, which may then be rotated around its longitudinal axis as the printhead ejects a series of droplets onto its surface, allowing an image to be formed thereon.
US 2011/0232514 A1 discloses an apparatus for printing on bottles wherein the bottles are carried in a horizontal plane with their longitudinal axes being held vertical during printing. A single electrostatic printhead prints onto each bottle whilst moving along substantially the same path as the bottle.
The geometry of this printing apparatus requires that each printhead is aligned with its ejection locations arranged along a vertical axis. A pressure gradient is likely to exist between the different ejection locations which will require a complex ink feed apparatus and calibration process to produce high quality images.
WO 2012/147612 A1 discloses a printing apparatus wherein cans are printed upon by a number of printheads while both the cans and the printheads are moved in conjunction through a vertical plane.
The plurality of orientations of the printheads as disclosed here, which are additionally subjected to accelerating forces as they follow a circular motion, are also likely to require a complex ink feed apparatus and calibration process in order to produce high quality images.
US 2013/0269551 A1 discloses a printing apparatus wherein bottles or cans mounted on a carrier with their principal axes vertical are moved horizontally between print stations. The print stations are moved vertically relative to the bottles or cans, to bring them into and out of the vicinity of one another.
This apparatus also comprises vertically aligned ejectors, which will suffer from the disadvantages described above.
WO 2012/131478 A2 discloses a printing apparatus in which a cylindrical objects are carried by holding devices through a single vertical path comprising multiple printhead stations.
U.S. Pat. No. 6,769,357 B1 discloses a can printing apparatus wherein cans are carried through a substantially circular path between a series of printhead stations. The apparatus discloses a number of printhead stations comprising printheads in different orientations. Such a system would require a complex ink feed system to maintain the correct ink pressure at the various ejection locations.
Also, in many cases it is not possible for the entire image to be formed by a single printhead during one rotation of the object. It may be the case, for instance, that the image is formed of several colours, each of which must be printed by a different printhead. It may also be the case that, in order to achieve the desired print resolution or density, each ejector is required to make several passes over the object. Also, if the longitudinal extent of the object is greater than the width of a single printhead, several printheads may need to be positioned in order to span the entire surface. Alternatively, the same printhead may be moved relative to the object over several passes.
This general inability of a single inkjet printhead to form a complete image on a cylindrical object during a single pass is one factor which limits the rate at which cylindrical objects can be printed upon. The other limiting factor is the maximum rate at which a single printhead can print, which is generally a fixed characteristic of the type of printhead used and may not be increased.
In order to overcome this limitation and, thereby, increase the throughput of a printing system, it is necessary to perform multiple print operations in parallel. This may be achieved by several printheads at a printhead station simultaneously printing upon the same object, or several printhead stations which simultaneously print upon different objects. In general, it is possible to have a series of printhead stations, each of which comprises a number of printheads, arranged such that at each printhead station a cylindrical object is being printed upon by several printheads. The cylindrical objects may then be carried from one printhead station to the next in order that different aspects of the image may be printed at the different printhead stations. Using this technique, the total rate of print operations occurring simultaneously can be increased from that possible using a single printhead by a factor of Np×Ns, where Np is the number of printheads at each printhead station and Ns is the number of printhead stations in total. While Ns is not limited, there is only sufficient space for a certain number of printheads, Np, to be arranged such that they are able to simultaneously print into the same object. Furthermore, there are several reasons why using the maximum number of printheads which may print onto the same cylindrical object is not necessarily the optimum arrangement.
A problem arises when multiple printheads are oriented differently to eject ink in different directions. The ink feed apparatus which feeds the printhead ejectors must be maintained at a fixed orientation in order to regulate the pressure and flow of ink correctly to the ejectors. Therefore having multiple printhead orientations requires a more complicated design of the ink feed system for each printhead, which can be oriented independently of the printhead, adding to its physical size and complexity. Another problem with this arrangement is that the pressure control of each ink feed must be set independently to account for the different hydrostatic pressure that results from the variable height between the ink feed and the printhead ejectors when the printhead is arranged in different orientations, adding complexity to the operation of the ink feed apparatus.
Furthermore, if the ejectors of a single printhead do not lie in the same horizontal plane, the ink pressure at each ejection location will vary, affecting the ink output across the printhead and the quality of the printed image.
A further problem occurs when printheads are oriented to eject ink at an angle above the horizontal, as dust and other airborne particles are likely to settle onto the printing face of the printhead and compromise the reliability of ejection.
Furthermore, as it is necessary for an object to be printed on to be carried from one printhead station to the next, it is desirable that the arrangement of printheads at each printhead station does not obstruct the preferred path of the objects or holding devices between the printhead stations. Were the cylindrical surface of the object to be surrounded on all sides by printheads, it would require a highly complex motion of its holding device to extricate it from a first printhead station and another complex motion to position it in a second printhead station, compromising throughput and making the accurate registration of print from station to station very challenging.
There is a need to provide an arrangement of printheads and printhead stations which provides as great a throughput of objects as possible, without compromising the effectiveness of the printhead operation or making the movement of the objects between printhead stations impractical.